Ecology & Environment
Stormwater Runoff Volume Calculator
Estimate effective rainfall, gross rainfall volume, runoff, storage demand, overflow, unused storage, average runoff rate, and captured fraction for one design event.
Rainfall after entered abstraction (mm)-
Gross effective rainfall volume (m³)-
Coefficient-adjusted runoff volume (m³)-
Runoff less controlled release (m³)-
Volume above entered storage (m³)-
Storage remaining after event (m³)-
Average event runoff flow (L/s)-
Decision view
Rainfall-runoff-storage flow and overflow threshold
Rainfall-runoff-storage flow and overflow thresholdEvent volume passes from effective rain through the runoff coefficient and release branch before filling storage and exposing overflow risk.
Exact scenario comparisonDesign rainfall depth (mm) changes while all other entered assumptions remain constant.
| Design rainfall depth (mm) | Rainfall after entered abstraction (mm) | Gross effective rainfall volume (m³) | Coefficient-adjusted runoff volume (m³) | Runoff less controlled release (m³) | Volume above entered storage (m³) | Storage remaining after event (m³) | Average event runoff flow (L/s) |
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How to use Stormwater Runoff Volume Calculator
- Enter catchment area, rainfall, and initial abstraction.
- Enter an event runoff coefficient.
- Enter storage, controlled release, and duration.
- Follow the rainfall-to-runoff flow and overflow threshold.
Calculator guide
Understanding Stormwater Runoff Volume Calculator
A rainfall event becomes runoff only after initial abstraction and a runoff coefficient, then controlled release and storage determine overflow. This calculator preserves every volume in that path and converts event volume to average flow.
Calculation method
How the calculation works
Convert effective rainfall depth over an entered drainage area to volume, apply a composite runoff coefficient, and reconcile release, storage, overflow, and average flow. Subtract initial abstraction from rainfall, convert millimetres over square metres to cubic metres, apply the runoff coefficient, subtract controlled release, and compare the remainder with storage.
Detailed calculation process
Route a design storm from rainfall depth to storage overflow
The defaults apply 45 mm rainfall to 3,200 m², subtract 3 mm abstraction, use coefficient 0.72, release 12 m³, and provide 75 m³ storage.
General formula: P_e = max(P-P_a,0); V_g = A P_e/1000; V_r = C V_g; V_d = max(V_r-V_rel,0); V_o=max(V_d-V_s,0); q_avg=1000V_r/(3600t) One millimetre over one square metre equals one litre, so division by 1,000 converts to cubic metres. Release reduces the event demand before storage; only demand beyond storage overflows.
What each symbol means
Worked substitution with the default inputs
The default storm creates 96.768 m³ runoff, leaves 84.768 m³ after controlled release, fills 75 m³ storage, and overflows 9.768 m³.
Event pathway
Trace every cubic metre from sky to overflow
The flow diagram sizes rainfall, runoff, release, stored water, and overflow as one reconciled event.
Worked situations
Practical examples
- Forty-two effective millimetres over 3,200 m² is 134.400 m³.
- Coefficient 0.72 produces 96.768 m³ runoff.
- Storage overflow is 9.768 m³.
Better inputs
Useful tips
- Choose coefficients and storms for the relevant surface and return period.
- Confirm whether release can occur during the event.
- Use hydrograph routing when timing and peak discharge matter.
Before relying on the result
Limitations and common mistakes
- This is a single-volume event balance, not a time-step hydrograph.
- Rainfall intensity distribution, routing, antecedent moisture, infiltration dynamics, pipe capacity, tailwater, and climate uplift are excluded.
- Drainage approval requires local standards and site modeling.
Reference
Key terms
- Initial abstraction
- Rainfall depth lost before runoff begins.
- Runoff coefficient
- Fraction of effective rainfall converted to runoff.
- Overflow
- Event demand remaining after release and available storage.
Important note
Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.
Frequently asked questions
Why divide rainfall by 1,000?
It converts millimetres to metres so area times depth produces m³.
Is average flow the peak flow?
No. It spreads event runoff evenly across the entered duration.
What happens if storage is larger than demand?
Overflow becomes zero and unused storage is reported.
Does release always happen before storage?
That is the calculator's simplified event accounting assumption.